This is a competitive renewal of NIH grant R01 AI52349-05. We achieved the major objective to develop a new model representing the first productive PERV-A infection in an animal by creating mice transgenic for the human PERV receptor, HuPAR-2. The primary objectives are: 1) to characterize the immune response to productive PERV infection - humoral and cellular, 2) to determine the potential that infection will produce leukemia or lymphomas, and 3) to advance our understanding of PERV envelope interactions with the receptor including mapping the Receptor Binding Domain (RED). Specific Aim #1: Characterize the productive PERV-A infection in the HuPAR-2 transgenic mice with a focus on three key features, the immune response to PERV, impact of immunosuppression and the tumorigenic potential of PERV. Hypothesis #1: A sustained exposure to PERV in irnmunocompetent animals results in both cellular and humoral immunity that eventually controls the infection. Hypothesis #2: Immunosuppressed animals will demonstrate an uncontrolled productive infection with defects in anti-PERV CTL and failure to develop blocking antibodies. Hypothesis #3: Productive PERV infection will increase the risks of malignancy, especially hematopoietic cell malignancies. Hypothesis #4: PERV antigens contribute to the immune response against transplanted pig tissues leading to rejection. Specific Aim #2: Identify and functionally characterize the viral and cellular determinants required for PERVA entry into cells. Hypothesis #1: Sequence motifs of the HuPAR-2 receptor required for PERV Env binding can be mapped with mouse/human chimeric receptor constructs. Hypothesis #2: Mapping the same for HuPAR-1 will closely match the results for HuPAR-2 providing a full picture of PERV virus/receptor interactions. Hypothesis #3: Specific sequence motifs in at least three regions of PERV Env (VRA, VRB, and the region C-terminal to the PRR) are required for virus entry, and, thereby, influence receptor specificity and tropism. The work proposed will advance our understanding of a new animal model for a gammaretrovirus and a novel viral receptor expressed widely in human tissues. It will contribute to a science-based approach to determining if risk of cross-species infection in xenotransplantation can be mitigated by strategies to create protective immunity.